FIG. 23A shows an outlook of a conventional driving device for an electric compressor. Power-lead wires 55 are coupled to a main battery, of which output is approx. DC 300V, built in an electric vehicle. An inverter-circuit-converting DC current supplied from the battery into AC current-is built in the driving device. A housing case is made of metal, and blocks electromagnetic waves radiating from inside to outside of the case as well as incoming from outside to inside of the case. The heat generated in the inverter-circuit dissipates into cooling water running through water-cooling pipe 56. FIG. 23B shows inside of the driving device, where circuit board 57 with electronic components, and electrolytic capacitor 64 are disposed. Inverter-module 60 and protective diode 63, both shown in FIG. 24, are coupled to circuit board 57. Protective diode 63 would block the current if power-lead wires 55 are coupled to the battery with the reverse polarity by mistake. Both of inverter-module 60 and diode 63 are mounted to a cooling mechanism for pipe 56.
FIG. 25 shows a circuit diagram of the electric vehicle. Battery 1 supplies current to motor driving device 4 and compressor driving device 5 via turn-on device 2. Device 4 drives drive-motor 62, and device 5 drives electric compressor 23. Drive-motor driving device 4 includes inverter-circuit 8 and electrolytic capacitor 3 which smoothes current to be supplied to inverter-circuit 8. Drive-motor 62 is coupled to inverter-circuit 8. Compressor-driving device 5 includes inverter-circuit 9, electrolytic capacitor 64 and protective diode 63. Capacitor 64 smoothes current to be supplied to circuit 9, and diode 63 blocks inverse current. Compressor 23 is coupled to inverter-circuit 9. Turn-on device 2 charges capacitors 3 and 64 up to the same voltage as battery 1 via charging-resistor 10, then supplies current from battery 1 to driving devices 4 and 5 via main relay 11.
FIG. 26 shows a circuit diagram of driving device 5 for the compressor. Air-conditioning controller 21, disposed outside device 5, calculates ability, e.g., an r.p.m. of compressor 23, and inputs the ability to microcomputer 19, which controls the inverter, via communication circuit 20. Control power supply 22 powers microcomputer 19, communication circuit 20, air-conditioning controller 21, and an audio and a navigation systems (not shown) and the like. Power supply 22 is insulated from battery 1, and battery 1 feeds power to power supply 22 via a DC—DC converter. The voltage-supplied from battery 1 and applied to driving device 5-is divided by upper voltage-dividing resistor 13 and lower voltage-dividing resistor 14. The voltage is then insulated by voltage-detector 16 and fed into microcomputer 19. The current running through inverter-circuit 9 is detected by current sensor 15 and insulated by current detector 17, then fed into microcomputer 19 which controls the inverter. Based on at least these inputs, microcomputer 19 sends a signal to gate-driving circuit 18, thereby activating a group of switching elements of inverter-circuit 9 for driving compressor 23. Gate-driving circuit 18 also insulates circuit 9 from microcomputer 19. Microcomputer 19 receives temperature data continuously from a thermistor-temperature-sensor of compressor 23 in addition to the inputs discussed above. Switching power supply 12 produces a power source for gate-driving circuit 18 and others. Current sensor 15 includes a coil and thus has an inductance component.
FIG. 27A shows a waveform of a current fed into inverter-circuit 9. FIG. 27B shows a waveform of a current fed into compressor-driving device 5. The waveform of current fed into inverter-circuit 9 shapes in a rectangular, and the current fed into compressor-driving device 5 shapes in a ripple-waveform, which however contains a constant current because the current to be fed into circuit 9 is smoothed by electrolytic capacitor 64.
A surge voltage generated in driving device 5 is described hereinafter with reference to FIGS. 28 and 29. FIG. 28 shows a circuit diagram where electrolytic capacitor 64 is deleted. Lead-wires from the power supply have inductance components 65. The current, of which waveform is illustrated in FIG. 29A, runs through inductance component 65 and flows into inverter-circuit 9. As a result, the surge voltage shown in FIG. 29C is generated when the current is turned off. In this case, if protective diode 63 exists, it would tend to block the energy of inductance component 65 from being released by the resonance between stray electrostatic capacitance on the power-line and the inductance component 65. As a result, the circuit is vulnerable to damage by the surge voltage. If electrolytic capacitor 64 exists in the circuit, the current flowing into inverter-circuit 9 would run as short as between capacitor 64 and circuit 9, thus the surge voltage is not generated as shown in FIG. 29B.
Conventional compressor-driving device 5 discussed above is desirably downsized and light-weighted in order to reduce the size and weight of electric circuits of an electric vehicle. Metallic cases, electrolytic capacitor 64 and protective diode 63 against a reversal connection are desirably removed; however, the removal of these elements would cause various problems.